I believe its because the material they use to make safari tanks lends itself to a more simple and cheap process called roto molding.

Decent materials like the oem that can be painted etc needs to be injection molded or blow molded and is a much more expensive process as is also the raw plastic material.

In other words, they are a small company and started out on the cheap, it worked and now they are raking in the buckets of our money.
So as far as they are concerned, why bother doing it any better and having to invest more money when they dont need to.

Shame, but thats life!

Its just a shame no one else did it either.

I think its more likley that they make the tanks themselves and dont want to farm it out, the rest of their stuff is all made in the same way, however you can roto mold polyimide in the same way as polythene.
In the uk about, £2-3000 for the tooling, x2 for both sides plus the cost of the design process, unless you can laser scan a set of safari ones (forgetting of course the copyright issues), so in the region of £6-10000 even before you get to actual manufacture.
I am tempted to get a set made in Ali even as a one off it should come out to close to the same cost of the safari tanks.